In recent years the liquid chromatography has been attracting attention of the people in the fields of chemistry, biology, medical science, etc., as one of the effective analyzing means.
The chromatography is aimed to, while delivering or supplying on one hand a solvent as mobile phase from a certain reservoir, by means of a supplying means such as a pump, to a separation means of the liquid chromatograph, for example a column, introduce on the other hand a sample, an object to be analyzed, into the separation means in order to develop and separate the sample with the aid of the mobile phase solvent for detecting the ingredients in the separated sample. In the liquid chromatography of this type, two or more different solvents have recently been utilized as the mobile phase, instead of the traditional method of employing one solvent, for the purpose of effectively eluting the aimed material from the separation means such as a column. Particularly the so-called gradient elution method capable of supplying the mixed solvents, while varying timewise the mixing ratio thereof in the course of supplying, to the separation means is given attention. A high performance liquid chromatography (HPLC), wherein the mobile phase solvent is rapidly supplied to the separation means under a high pressure of more than 50 Kg/cm.sup.2 and sometimes exceeding 100 Kg/cm.sup.2, is also developed for meeting the need or demand from some special uses requiring to effectively and speedily separate even a small amount of sample.
A liquid chromatograph for carrying out liquid chromatography of this kind is required as the apparatus therefor to have (1) ability to gradient elution capable of varying the mixing ratio of the solvents as a function of time accurately and freely, (2) constancy of the flow rate of the mobile phase solvent and delivery (transportation) thereof without pulsation to the separation means, and (3) capability of resisting high pressure, etc.
Various types of liquid chromatographs have been proposed for satisfying the above-mentioned conditions, in particular mobile phase supplying means of various kinds. Among them a liquid chromatograph employing a reciprocation type plunger pump for delivering or supplying the mobile phase is worthy of attention as a prior art in respect of its economical property.
Nevertheless it is far away from being ideal, still containing inherent problems to be solved. For supplying the mobile phase under low pulsation with a single plunger pump, a method of regulating the discharge amount constant, by controlling the speed of the plunger reciprocation, by means of shortening the time duration of suction on one hand and elongating the discharge time duration on the other hand, is generally adopted. This rapid suction of the mobile phase places the mobile phase on the suction side under pressure reduction, with a result of facilitating or accelerating the gas desolved in the mobile phase solvent to evaporate for forming bubbles, that is known as cavitation. It consequently degrades the accuracy (constancy) of the flow rate of the mobile phase solvent. It is required particularly in the gradient elution to operate the valves for sucking the individual solvents constituting mobile phase from the respective reservoir in such a short time, for example 0.1-0.2 second, wherein the plunger pump carries out the sucking of the solvent. Considering the response time required of the valves such as 10-20 msec., the operational error may be noticeably large, making the mixing of the mobile phase solvent (mixed solvent) at an accurate ratio difficult. Some attempts for solving the problem are seen, for example U.S. Pat. No. 3,985,021 and others, wherein electronic computer controlling, electric controlling system, etc. are adopted. In those propositions, controlling systems are mostly very complicated, leading the manufacturing cost very high as a whole. Otherwise each solvent reservoir must be equipped with an individual pump, which means plural pumps are required after all, resulting in high manufacturing cost and in difficulty of getting exact regulation of accurate mixing ratio of the plural solvents.